1. Field of the Invention
The present invention relates to a membrane filtration system which is equipped with a filtering membrane portion for removing impurities in feed water and which returns a part of concentrate from the filtering membrane portion to an upstream side of the filtering membrane portion and discharges a remainder of the concentrate.
2. Description of the Related Art
As disclosed in JP 05-220480 A, as a water treatment system for feed water to an apparatus, a membrane filtration system is available which has a filtering membrane portion for filtering impurities contained in feed water, such as dissolved salts. In this membrane filtration system, feed water from a feed pump flows into the filtering membrane portion, and impurities contained in the feed water are filtered out. Then, permeate flowing out of the filtering membrane portion is supplied to the apparatus.
Apart from the permeate, concentrate flows out of the filtering membrane portion. As an example of the membrane filtration system, there exists a cross flow filtration type membrane filtration system which discharges only a part of the concentrate, and returns the remainder to the upstream side of the feed pump through a concentrate return line. In this membrane filtration system, in order to prevent fouling, that is, deposition on or adhesion to the membrane surface of suspended substances, colloids, organic matter, etc. in the water, operation is conducted so as to secure a predetermined flow velocity at the surface of the filtering membrane. For this purpose, the feeding flow rate of the feed pump is set such that the flow rate of the concentrate from the filtering membrane portion is of a fixed value or more with respect to the permeate flow rate, that is, the flow rate of the product water. However, when the concentrate flow rate increases to a degree more than necessary with respect to the flow rate of the product water, the feeding flow rate of the feed pump also increases, resulting in an increase in power consumption. In view of this, the concentrate flow rate is set so as to be of a predetermined proportion with respect to the flow rate of the product water so that no fouling may be generated and no wasteful power consumption may occur in the feed pump.
In this connection, the present applicant has proposed, in JP 2006-305499 A, a membrane filtration system operating method in which, in order to prevent clogging in the filtering membrane in the filtering membrane portion and to prevent discharge of concentrate in an amount more than necessary, the discharge flow rate of the concentrate is adjusted based on the temperature of one of the feed water to the filtering membrane portion, the permeate from the filtering membrane portion, and the concentrate from the filtering membrane portion, or on the quality of the feed water to the filtering membrane portion. However, if, upon an increase in the discharge flow rate of the concentrate, the return flow rate of the concentrate remains the same, the flow rate obtained by combining the discharge flow rate and the return flow rate, that is, the concentrate flow rate, becomes higher than the set value. As a result, the concentrate flow rate becomes higher than necessary as compared with the flow rate of the product water, and the feeding flow rate of the feed pump becomes higher than necessary, resulting in a wasteful power consumption. On the other hand, if, upon a reduction in the discharge flow rate of the concentrate, the return flow rate of the concentrate remains the same, the concentrate flow rate becomes lower than the set value. As a result, as compared with the flow rate of the product water, the flow rate of the concentrate is reduced, and the flow velocity at the surface of the filtering membrane decreases, so there is a fear of generation of clogging in the filtering membrane due to fouling.
Apart from this, the present applicant has proposed, in JP 2005-279459 A, a membrane filtration system operating method in which, in order to efficiently effect the filtration of impurities and the degassing of dissolved gas in correspondence with a change in the temperature of the feed water, the flow rate of the product water from the filtering membrane portion is adjusted based on the temperature of the feed water. However, if, upon a reduction in the flow rate of the product water, the return flow rate of the concentrate remains the same, the flow rate of the concentrate becomes higher than necessary with respect to the flow rate of the product water, and the feeding flow rate of the feed pump becomes higher than necessary, resulting in a wasteful power consumption. On the other hand, if, upon an increase in the flow rate of the product water, the return flow rate of the concentrate remains the same, the concentrate flow rate is reduced with respect to the flow rate of the product water, and the flow velocity at the surface of the filtering membrane decreases. As a result, there is a fear of generation of clogging in the filtering membrane due to fouling.